Wilson Disease (WD) is an autosomal recessive, potentially fatal disease caused by mutations in the ATP-dependent copper transport protein, ATP7b. Over 300 mutations have been reported to cause disease, the incidence is estimated to be 1 in 30,000 births, and the disease typically presents in the first or second decade of life. Under normal conditions, ATP7b mediates the incorporation of 6 Copper (Cu) molecules into apoceruloplasmin, thereby forming ceruloplasmin. Ceruloplasmin is secreted into plasma, and excess Cu is deposited into lysosomes (again by ATP7b) for excretion into bile. The result of defective ATP7b function is accumulation of Cu in the liver, brain, cornea, and other organs. The mechanisms by which Cu accumulation causes liver injury have not yet been fully elucidated. Examination of liver specimens from patients with WD and Bedlington terriers with Cu overload revealed increased lipid peroxidation and mitochondrial Cu when compared to control liver. The classical dogma is that excess copper overwhelms the natural detoxifying functions of metallothionien and glutathione, leading to oxidative stress with resultant mitochondrial toxicity, inflammation which then precipitates fibrosis/cirrhosis. Counter current to this paradigm is that although the underlying genetic defect is similar and hepatic copper is always elevated in WD, disease manifestations vary, and either a hepatic, neurologic, or psychiatric phenotype may dominate. Hepatic presentations range from mild inflammation to hepatitis, cirrhosis, and acute liver failure. The molecular mechanisms behind this phenotypic variability are not known. In addition, the diagnosis and treatment of WD remain a challenge for clinicians. Currently, copper-chelation is available as a life-long therapy for WD. However, side effects (observed in more than 75% of patients) (4), poor compliance due to a very slow body response (6-46 months), and risk of neurologic decompensation complicate therapy (5). A better mechanistic understanding of WD is needed to improve both the diagnosis and treatment of this lethal disorder.
ATP7b−/− mice are an established model with which to investigate the mechanisms of pathogenesis in WD (6). Similar to the human disease, the ATP7b−/− mice accumulate copper in the liver, show lack of copper incorporation into ceruloplasmin, marked decrease of oxidase activity in plasma, and elevated copper in the urine. The mice achieve maximum concentrations of hepatic copper at 6 weeks, yet these mice do not have significant liver pathology until after 12-14 weeks (6). In previous studies of ATP7b−/− mice, transcriptional analysis was not consistent with widespread oxygen radical-mediated damage, rather, we found that lipid metabolism is the process most significantly affected by copper overload, even before significant histologic changes in the liver are present (7). Our bioinformatics studies indicated that the observed down-regulation of cholesterol biosynthesis is due to inhibition of the signaling events mediated by the nuclear liver X receptor (LXR). LXR is part of the superfamily of ligand dependent, nuclear receptor transcription factors. Oxidized derivatives of cholesterol (oxysterols) are the natural ligands of LXR, and have the ability to both agonize and antagonize LXR activation (8). LXRα (encoded by NR1H3) is highly expressed in the liver, macrophages, and other highly metabolic tissues, whereas LXRβ (NR1H2) is ubiquitously expressed (for detailed review see (9)). Upon ligand activation, LXRs form a heterodimer with the retinoid X-receptor (RXR), and play a critical role in modulation of lipid metabolism and inflammatory signaling (10). Furthermore, nuclear receptor agonists that affect cholesterol biosynthesis, transport, and bile acid metabolism have emerged as a potentially revolutionary breakthrough in the treatment of non-alcoholic fatty liver disease and cholestatic liver disease (11).
To further understand the relationship between elevated hepatic copper, down-regulated lipid metabolism, and liver disease, we correlated our genetic findings in ATP7b−/− mice with data from WD patients. We further investigated the impact of LXR activation in this mouse model of WD. Here we outline a previously unrecognized role for nuclear receptors in the pathogenesis of this complex human disease. These observations identify nuclear receptor activation as a potential target for novel therapeutic strategies in WD.